undercutter / bed lifter - how thick and which steel to use?

[wwwfarmer]

Hello everyone!

I am planning to build an undercutter/bed lifter tractor attachment. The goal is to slide a slanted board 10-12inch (25-30cm) deep into the ground and lift the soil just enough, so that you don't need to use a shovel to dig out root crops such as garlic or carrots. This is how it looks in action:

And these are the dimensions (top link arm isn't drawn)

I am unable to asses how thick should the sidewalls and the bottom lifting board be and which steel is best suited for such applications. Can you help me out? Does anyone have an undercutter on the farm and would be willing to measure how thick the steel is?

Thank you for your help!

 

[Rustyiron]

Sounds like a good application for a length of a bucket cutting edge.
They come with a beveled edge and are hardened. A 84" edge could give you 2-2' "verticals" and a 3' horizontal for cutting. I'd be thinking about at least a 5/8 x 6" edge.
Your local heavy equipment or Bobcat type dealer should have a good selection.

 

[mikester]

I'd start with 1/4" AR400. It will be easier for your tractor to pull. The stuff is very tough and resists bending.

 

[bdog]

How much do you intend on using it? Higher grades of steel will last much longer in abrasive conditions but are costly. Regular old steel will do the job but won’t have as long of a lifespan. If it is just for your personal garden I would probably just use regular steel. If it is some type of commercial or larger scale operation I would look into something like hardox. I think the 5/8” x 6” is a good starting point. Might want to up it to 3/4” if pulling it with a larger tractor.

 

[wwwfarmer]

Thank you all for taking your time

I've contacted bulldozer reseller companies around me, and it amazes me how the cutting edge is literally twice the price as in the USA. Gotta pay the VAT I guess.

So my choice is between regular steel and hardened steel. I believe regular steel is marked as S355 or S235, the number 355 and 235 corresponding to the yield strength in MPa of the steel. Yield strength is determined under axial stretching of the specimen and corresponds to the maximal Force per area under which elastic deformation occurs. Hardened steel has yield strength 3-5 times higher than the regular steel. For example AR400 or Hardox 450 steels have yield strength 1187MPa and 1250MPa respectively.

My next question is, is yield strength directly proportional to how much the steel would resist bending in transverse direction (as opposed to axial)? Because it is nuts to think that 1/4" AR400 would resist more to bending than 3/4" S355 structural steel. Am I reading this numbers correctly?

 

[mikester]

Yes the stuff is really tough. You will need a very HD press brake if you are making bends. I wouldn't bother with cutting edges in your application.

 

[JohnnyMX]

Can you just use regular steel and hard face the leading edges that will make the most contact. I hard-faced my cheap box blade ripper tips and they after a few seasons I see it was well worth it compared to the one I left un treated. If you are digging through a garden bed, I can't imagine it is going to be too brutal for the steel unless you garden in a rock quarry.

 

[Rustyiron]

If the OP has beautiful soil with no sub surface suprises and not dragging it 8 hours a day I'd use "regular" steel and not even bother with the hard facing. It may even be a good idea giving him a inexpensive prototype. I'd imagine that there's a "sweet spot" for the angles of the 3 pieces that engage the soils so that it digs in just right.

 

[ptsg]

I've bought a cutting edge for another project and I didn't found it to be crazy expensive.

If I remember right, I paid around 60€ for 1 meter long, 150 mm wide and 16 mm thick cutting edge here in Portugal.

Back to the OP's project, this is one of those projects were an hydraulic top link helps a lot on adjusting just the right angle for the cutting edge.

 

[Ford850]

You might find a cheap solution to be a used rear dirt scoop. Cut the back end out of the bucket, and you're done! Used dirt scoops are usually sold for reasonable prices too.